The thermal shape memory effect in polymeric materials refers to the ability of a sample to retain a deformed shape when cooled below <i>T<sub>g</sub></i>, and then recover its initial shape when subsequently heated. Although these properties are thought to be related to temperature-dependent changes in network structure and polymer chain mobility, a consistent picture of the molecular mechanisms which determine shape memory behavior does not exist. This, along with large differences in the shape memory cycling response for different materials, has made model development and specific property optimization difficult. In this work we use coarse-grained molecular dynamics (MD) simulations of the thermal shape memory effect to inform micro-macro relationships and systematically identify the salient features leading to desirable shape behavior. We consider a simulation test set including chains with increasing levels of the microscopic restrictions on chain motion (the freely-jointed, freely-rotating, and rotational isomeric state chain models), each simulated with both the NPT and NVT ensembles. It is found that the NPT ensemble with attractive interactions between monomers enabled is the most appropriate for simulating the temperature-dependent mechanical behavior of a polymer using coarse-grained MD. Of the different models, the freely-jointed chain system shows the most desirable shape memory characteristics; this behavior is attributed to the ability of the particles in this system to pack closely together in an energetically favorable configuration. A comparison with experimental data demonstrates that the coarse-grained simulations display all of the relevant trends in mechanical behavior during constant strain shape memory cycling. We conclude that atomistic detail is not needed to represent a shape memory polymer, and that multi-scale modeling techniques may build on the mechanisms embodied in the simple coarse-grained model.
Ionic polymer transducers (IPTs) are soft sensors and actuators which operate through a coupling of micro-scale
chemical, electrical, and mechanical interactions. The use of an ionic liquid as solvent for an IPT has been
shown to dramatically increase transducer lifetime in free-air use, while also allowing for higher applied voltages
without electrolysis. In this work we model charge transport in an ionic liquid IPT by considering both the
cation and anion of the ionic liquid as mobile charge carriers, a phenomenon which is unique to ionic liquid
IPTs compared to their water-based counterparts. The electrochemical behavior of the large ionic liquid ions is
described through a modification of the Nernst-Planck equation given by Kornyshev which accounts for steric
effects in double layer packing. The method of matched asymptotic expansions is applied to solve the resulting
system of equations, and analytical expressions are derived for the nonlinear charge transferred and capacitance
of the IPT as a function of the applied voltage. The influence of the fraction of mobile ionic liquid ions and steric
effects on the capacitance of an ionic liquid IPT is shown and compared to water-based IPTs. These results show
the fundamental charge transport differences between water-based and ionic liquid IPTs and give considerations
for future transducer development.
In this work a model of ion transport in ionic liquid-based ionic polymer-metal composites (IPMC's) is formulated using
Nernst-Planck/Poisson (NPP) theory and numerical simulations are performed using the finite element method. IPMC's
are smart materials which act as both sensors and actuators, and the use of an ionic liquid has been shown to
dramatically increase transducer lifetime in free-air use while also allowing for higher applied voltages without chemical
decomposition. We consider both the cation and anion of the ionic liquid to be mobile in addition to the mobile countercation
of the transducer. The results show a nonlinear dynamic response which gives insight into transduction
mechanisms which are unique to ionic liquid IPMC's as compared to their water-based counterparts.